As a rewritable optical recording medium, a magneto-optical recording medium has practical application, but a conventional magneto-optical recording medium has an disadvantage such that its reproducing characteristics become worse as its recording bit diameter and recording bit gap become smaller than a beam diameter of a light beam to be irradiated. Such a disadvantage is due to that an adjacent recording bit comes into a converged light beam spot and thus respective recording bits cannot be reproduced separately.
In order to eliminate the above disadvantage, some announcements about a magneto-optically reproducing technique utilizing a magnetic super resolution phenomenon were made at MORIS '94. The abstracts of the announcements include No. 29-K-04 "MSR Disks with three Magnetic Layers Using In-Plane Magnetization Films" (p.125) and No. 29-K-05 "Magnetically-Induced Super Resolution Using Magneto-Static Coupling" (p. 126). In these abstracts, a magneto-optical recording medium in which an intermediate layer having in-plane magnetization or a non-magnetic intermediate layer is provided between a reproductive layer that are in-plane magnetization state at room temperature and that are in a perpendicular magnetization state at a higher temperature and a recording layer is used. As a result, a front mask and a rear mask in the in-plane magnetization state are formed, and a signal shows an abrupt change due to the rear mask.
In addition, No. 29-K-06 "New Readout Technique Using Domain Collapse on Magnetic Multilayer" (p. 127) disclosed that when satisfactory Jitter characteristics are obtained in an abrupt change in a signal due to the rear mask and when a reproductive signal is differentiated, a position of recording bits can be accurately detected.
In addition, the inventor makes a suggestion in connection with the magneto-optical reproducing technique utilizing the magnetic super resolution phenomenon as follows. FIG. 25 shows a schematic arrangement of the magneto-optical recording medium. As shown in FIG. 25, a substrate 71, a transparent dielectric layer 72, a reproductive layer 73, a non-magnetic intermediate layer 79, a recording layer 74, a protective layer 75, and an over coat layer 76 are laminated on a disk main body 200 in this order. The substrate 71 is made of a transparent base material, such as polycarbonate, and the substrate 71 has a disk shape.
The recording layer 74 has recording bits 201 and 202 to which digital information is recorded in perpendicular magnetization directions that are antiparallel. The recording bits 201 and 202 are magnetic domains used for recording information.
The reproductive layer 73 is provided on the recording layer 74 so as to have a reproductive bit on which a magnetization direction is transferred from the recording bits 201 and 202. The reproductive layer 73 is formed such that a compensation temperature of a perpendicular magnetization film of the reproductive layer 73 becomes closer to room temperature, saturation magnetization becomes strong at a higher temperature so as to become maximal in the vicinity of a reproducing temperature and that lowering of coercive force becomes less compared to increase in the saturation magnetization as the temperature rises from room temperature to the reproducing temperature. More specifically, a width of a stable magnetic domain, which can stably exist, in a reproducing bit becomes larger than a magnetic domain width 74a of the recording bits 201 and 202 at room temperature. Meanwhile, when the temperature of the reproductive layer 73 rises up to the reproducing temperature by a light beam 78 for detecting a magnetization direction of the reproductive bit, the stable magnetic domain width becomes smaller as the temperature rises so as to become smaller than the magnetic domain width 74a.
The non-magnetic intermediate layer 79 blocks magnetic moment of two magnetic ions, namely, exchange interaction that is magnetic coupling force for determining a relative direction of spinning between the recording layer 74 and the reproductive layer 73.
In accordance with the above arrangement, since the stable magnetic domain width of the reproducing bit on the reproductive layer 73 is larger than the magnetic width 74a of the recording bits 201 and 202 at room temperature, the reproducing bits cannot be exist with them having the same width as of the recording bits 201 and 202. Moreover, since the exchange interaction between the reproductive layer 73 and the recording layer 74 is blocked by the intermediate layer 74, the magnetization direction of the reproducing bit does not become same as that of the recording bits. When the converged light beam 78 is irradiated onto the reproductive layer 73, the reproductive layer 73 and the recording layer 74 have the same temperature distribution according to intensity distribution of the light beam 78 (approx. Gauss distribution). As the temperature rises, the stable magnetic domain width of the reproducing bit becomes small, and when the reproducing bit having a size corresponding to the recording bit 201 can exist stably, the reproducing bit 73a whose magnetization was reversed by a stray magnetic field 207 generated from the recording layer 74 is formed. At this time, since the magnetic coupling force between a reproducing bit 73b other than the reproducing bit 73a and the recording layer 74 is small, the reproducing bit 73b is separated from the reproducing 73a, and its magnetization direction becomes same as that of the recording bit due to an external magnetic field and the like.
Therefore, when the reproductive layer 73 is suitably set, the magnetization of only one portion of the domain to which the light beam 78 was irradiated can be reversed. For this reason, even if a size and gap of the recording bits 201 and 202 are decreased, the reproducing bits can be separated independently, thereby improving recording density of a magneto-optical recording medium. However, in the magneto-optical recording medium announced in the MORIS '94, the falling of the reproductive signal becomes abrupt due to the rear mask formed at the time of the reproduction, but the rising of the reproductive signal becomes gentle as the light beam moves and as the temperature rises like the conventional one. For this reason, the recording density of the magneto-optical recording medium cannot be sufficiently improved.
In addition, in the magneto-optical recording medium suggested by the inventor of the present invention, just when the temperature of the reproductive layer 73 rises and the stable magnetic domain width becomes smaller than the magnetic domain width of the recording bits 201 and 202, the reversed magnetic domain is instantaneously formed on the reproductive layer 73, and thus the rising of the reproductive signal becomes abrupt. However, since the temperature of the reproductive layer 73 is comparatively gently lowered, the falling of the reproductive layer becomes gentle. Therefore, the recording density of the magneto-optical recording medium cannot be sufficiently improved.